1. Field of the Invention
The present invention relates to a slip control system for a vehicle and, more particularly, to a slip control system for the vehicle, adapted to prevent a slip of the driven wheels on a road surface from becoming excessive by controlling torque to be applied to the driven wheels.
2. Description of Related Art
Prevention of a slip of the driven wheels from becoming excessive is effective in order to efficiently provide a propulsive force of the vehicle as well as to provide safety in terms of prevention of a spin of the vehicle body and so on. In order to prevent an excessive amount of a slip of the driven wheels, torque to be applied to the driven wheels as a cause of slipping may be reduced.
Slip control of this type is disclosed, for example, in U.S. Pat. Nos. 4,484,280 and 4,583,611 (corresponding to Japanese Patent Unexamined Application (kokai) Nos. 16,948/1983 and 56,662/1985, respectively). T They disclose technique of reducing the torque applied to the driven wheels by braking the driven wheels by brakes as well as reducing the torque to be generated by the engine. More specifically, U.S. Pat. No. 4,484,280 (corresponding to Japanese Patent Unexamined Application (kokai) No. 16,948/1983) discloses the technique of reducing the torque to be applied to the driven wheels only by braking the driven wheels when a slip of the driven wheels is small and by reducing the torque generated by the engine, on top of braking the driven wheels, when a slip of the driven wheels becomes larger. U.S. Pat. No. 4,583,611 (corresponding to Japanese Patent Unexamined Publication (kokai) No. 56,662/1985) discloses the technique in which, when one of the left-hand and right-hand driven wheels is larger in slippage than the other, only the driven wheel having a larger slip value is braked and, when both of the driven wheels are larger in slippage, the two driven wheels are braked while the torque to be generated by the engine is reduced. In summary, the technique as disclosed in the two prior patent publications as described hereinabove uses the braking of the driven wheel or wheels by the brake or brakes as a main means and the reduction in the torque generated by the engine as an auxiliary means.
Japanese Patent Unexamined Publication (kokai) No. 31,869/1988 proposes that an excessive slip be reduced without delay by temporarily reducing torque applied to the driven wheels by a predetermined decrement when the slip value of the driven wheels becomes larger than a given judgment value larger than a predetermined target value on condition that feedback control is implemented in order to make the slip value of the driven wheels the predetermined target value. It has been found, however, likely to cause the problem that implementation of the feedback control toward the target value and a temporary reduction of the torque applied by the given decrement when exceeding the given judgment value may cause the torque applied to the driven wheels to be reduced to an extent that is more than necessary, thus impairing an acceleration feeling.
More specifically, when the feedback control is implemented so as to cause the slip value of the driven wheels to reach the target value, a control gain should be made small if a stable control is taken into account. Thus, if the control gain would be made small, the problem may arise as to responsiveness of control. This point can be offset by temporarily reducing the torque by a given decrement when the slip value exceeds the predetermined judgment value. In this case, however, there may often arise occasions where a temporary reduction in the torque applied becomes to great and, even if the feedback control would be implemented to reach the target value from the state in which the torque applied is reduced too much, a return to the target value requires a longer time period and this may result in worsening of the acceleration feeling.
If a temporarily reduced amount of the torque is made small in order to prevent the acceleration feeling from worsening, this may cause the problem in terms of rapid convergence of a slip.